You can write a bit banging CAN driver, but it has many limitations.
First it's the timeing, it's hard to achieve the bit timing and also the arbitration.
You will be able to get 10kb or perhaps even 50kb but that consumes a huge amount of your cpu time.
And the code itself is a pain.
You have to calculate the CRC on the fly (easy) but to implement the collision detection and all the timing parameters is not easy.
Once, I done this for a company, but it was a realy bad idea.
Better buy a chip for 1 Euro and be happy.